The invention relates to a reference ladder providing a plurality of reference signals having an extremely high accuracy.
Some electronic devices require a large number of high accuracy reference signals. These signals are usually obtained by means of a chain of equally valued resistors coupled between a reference voltage source and ground. If the resistor ladder is referenced to a variable signal source rather than ground, a reference current source is required
For instance, conventional flash analog-to-digital converters comprise a reference voltage source, comparators connected in parallel and a chain of equally valued resistors providing reference voltages thereto. The input bias currents of comparators with bipolar input stage are very high and vary with the input signal. The bias currents are added so that different resistors of the resistor ladder conduct different currents. Comparators with FET input stage have a very poor accuracy. Autozeroing the comparators is necessary, whereby large parasitic capacitances are introduced. As a result, reference signals provided by the resistor ladder are unstable and resistor values must be small.
The resistor ladder causes deficient long term and temperature stability, reduced speed and accuracy, enlarged chip space and increased power consumption, etc. In contrast, integrated MOS capacitors are most accurate integrated circuit components. For instance, their absolute accuracy and tolerance fluctuation due to time, mechanical stress and voltage are far superior to any other component.